1) Field of the Invention
The present invention relates to a controller for an interleaved power converter, and an interleaved power converter which incorporates such a controller.
2) Description of the Prior Art
A single phase boost converter comprises an inductor connected between a DC input voltage and a switch. The switch alternatively connects the inductor to the input voltage and to an output, and is driven at a particular duty cycle. The circuit provides an output voltage which is always greater than or equal to the input voltage. A buck converter comprises a similar circuit operating in reverse so that the input voltage is always greater or equal to the output voltage.
As the inductor in a boost converter is continuously charging and discharging, the resulting inductor current has an AC component termed a ripple current. Generally, such ripple currents are undesirable as they degrade component performance and introduce unwanted effects into the circuit.
One of the known ways of reducing ripple currents is to operate two or more converter circuits (sub-circuits) in parallel, and to operate the switches in the respective sub-circuit with a phase shift (ie a phase difference) with respect to each other. The phase difference between the operation of the two switches results in the ripple currents of one of the sub-circuits cancelling the ripple currents of the other. This reduces the ripple current in both the input and the output of the converter.
Such a circuit is known as an interleaved boost converter. A first example of a 2-phase interleaved boost converter used for power-factor correction (PFC) is illustrated in FIG. 1.
A second example of a two phase boost converter is illustrated in FIG. 2. The converter of FIG. 2 is known as a 3-state switching cell. The converter of FIG. 2 is similar to that of FIG. 1, except that the inductors from each sub-circuit are magnetically coupled together in the form of a transformer, and a storage inductor is connected between the voltage input and the transformer.
The circuits of FIGS. 1 and 2 have essentially the same transfer functions and operate in continuous mode with essentially the same duty-cycle.
The converter of FIG. 2 reduces ripple currents flowing through individual components of the circuit as compared with the circuit of FIG. 1. Moreover, the circuit of FIG. 2 displays lower peak currents in the switches when compared to conventional power conversion circuits, and this reduces switching losses.
As is well understood, the control of a single-phase boost converter, such as that described in Freescale application note An3843: Single Phase Two-Channel Interleaved PFC converter Using MC56F8,0006 Rev 0, 04/2009 usually requires three signals to be sensed. Namely, the output voltage of the converter Vout, the DC input voltage Vin (the DC output of a bridge rectifier may conveniently be regarded as the input voltage for this purpose), and the input current I (the current flowing in the common return line to the bridge rectifier may conveniently be sensed for this purpose). The sensed values are processed by a controller, and used to alter the operating conditions to maintain the desired output.
When multiple phases are interleaved, current imbalances between the phases may occur due to variations in performance between the components of the different sub-circuits. It is desirable to adjust for these current imbalances, particularly with converters of the type illustrated in FIG. 2, where current imbalance can lead to transformer saturation, increased RMS currents and reduced reliability.
However, it is not possible to determine the current for each phase (ie the current flowing in the switch of each sub-circuit) from the input current. For this reason, known interleaved power converters sense the current in each switch directly, in order to determine and adjust for current imbalance. See for example the Freescale reference above. The sensed current for each phase may be compared to a common threshold, or to two different thresholds, which allows the controller to identify and adjust for any imbalances.
However, this requires additional current sensing, as compared with the single phase converter, and thus adds to the cost and complexity of the controller.